1. Field of the Invention
The present invention generally relates to an X-ray beam limiting apparatus for projecting X-ray beams emitted from two X-ray focal points separated by a predetermined distance, toward a common projection field at an X-ray detecting plane. More specifically, the present invention is directed to an X-ray beam limiting apparatus used for stereoscopic radiography and fluoroscopy, in which pyramid-shaped X-ray beams emitted from two X-ray focal points narrowly separated with each other are effectively limited by a pivotable blade.
2. Description of the Related Art
Conventionally, stereoscopic radiography or fluoroscopy has been widely utilized with employment of the X-ray tube having two separate X-ray focal points. In performing such a stereoscopic radiography or fluoroscopy, X-ray beams emitted from the respective focal points of the X-ray tube are alternately irradiated or projected toward a biological body, e.g., a patient, and the X-ray beams which have passed through this biological body are detected by either the X-ray film, or image intensifiers. Then, based on the X-ray detection information obtained by this X-ray detecting device, one X-ray image of the biological body produced from the X-ray beams emitted from one X-ray focal point is observed by a righthand eye of a viewer, whereas the other X-ray image thereof produced from the X-ray beams emitted from the other X-ray focal point is observed by a lefthand eye of this viewer, whereby the X-ray image of the biological body may be stereoscopically observed by the viewer.
The above-described stereoscopic radiography, or fluoroscopy is known in the art, for instance, U.S. Pat. No. 4,766,603 issued to Okabe, entitled "APERTURE DEVICE OF RADIATION DIAGNOSTIC APPARATUS" on Aug. 23, 1988. FIG. 1 is a schematic sectional view of the conventional X-ray beam limiting apparatus 10 disclosed in the above-described U.S. Pat. No. 4,766,603, and also FIG. 2 is an explanatory diagram for representing operations of this X-ray beam limiting apparatus 10.
In FIG. 1, a stereoscopic X-ray tube 15 has two X-ray focal points "R.sub.1 " and "L.sub.1 " separated from each other. The above-described X-ray beam limiting apparatus 10 is mounted at the X-ray projection side of this X-ray tube 15. The X-ray beam limiting apparatus 10 is so constructed that pyramid-shaped X-ray beams "XR.sub.1 " and "XR.sub.2 " emitted from the respective X-ray focal points "R.sub.1 " and "L.sub.1 " of the stereoscopic X-ray tube 15 are alternately projected to a common projection field "S.sub.1 " at an X-ray detecting plane "I.sub.1 " under beam limiting effects.
Normally, such an X-ray stereoscopy is carried out under condition that a biological body "P" under medical examination is positioned in close contact with this X-ray detecting plane "I.sub.1 " and therefore is so-called as a "contact X-ray stereoscopy, or fluoroscopy". In accordance with the feature of this contact X-ray stereoscopy, since the X-ray images of the biological object are detected with approximately 1:1 correspondance, the distance between two X-ray focal points "R.sub.1 " and "L.sub.1 " is selected to be approximately 63 mm equal to a distance between human eyes.
The construction of this conventional X-ray beam limiting apparatus 10 will now be described.
The X-ray beam limiting apparatus 10 includes first rectangular blades 16 and 17 arranged in a triangular space "V.sub.1 " defined by two inside edges "XR.sub.1a " and "XR.sub.2a " of the pyramid-shaped X-ray beams "XR.sub.1 " and "XR.sub.2 ", and also a line "RL.sub.1 " connecting two separate focal points "R.sub.1 " and "L.sub.1 ". The first rectangular blades 16 and 17 are positioned at the X-ray projection side of the stereoscopic X-ray tube 15 and have a function to limit these X-ray beams. Furthermore, a circular fixed blade 18 is employed below the first rectangular blades 16 and 17 along the X-ray beam travel path so as to shield the X-ray beams which would exceed an outer case (not shown) of the X-ray detecting plane "I.sub.1 ". Also, a compensating filter blade 19 is similarly positioned below the circular fixing blade 18 in order to compensate for differences in X-ray absorption occurring in a heart and a lung. The conventional X-ray beam limiting apparatus 10 further employs second rectangular blades 20 and 21 arranged below the compensating filter blade 19 to set the rectangular X-ray projection field; third rectangular blades 22 and 23 also arranged below the second rectangular blades 20 and 21 and outside the outer edges "XR.sub.1b " and "XR.sub.2b " of the pyramid-shaped X-ray beams "XR.sub.1 " and "XR.sub.2 " in order to beam-limit the outer edges "XR.sub.1b " and "XR.sub.2b "; an outer cover 14; stepping motors (not shown in detail) each for horizontally moving the above-described blades 16, 17, 19 to 23 except for the circular fixing blade 18; and also a controller (not shown) for controlling rotations of these stepping motors.
A distance "SID" (abbreviated "X-ray Source To Image Receptor Distance") among the X-ray focal points "R.sub.1 ", "L.sub.1 " and the X-ray detecting plane "I.sub.1 ". In this case, the controller transfers a predetermined number of rotation pulses to the respective stepping motors for driving the corresponding beam-limiting blades 16, 17, 19 to 23 in such a manner that a desirable common projection field "S.sub.1 " defined at the X-ray detecting plane "I.sub.1 " may be established by the beam-limited pyramid X-rays "XR.sub.1 " and "XR.sub.2 " emitted from the X-ray focal points "R.sub.1 " and "L.sub.1 ".
As is known in the stereoscopic radiography or fluoroscopy, highspeed continuous photographing operations such as several X-ray film sheets per one second and several tens of images per one second for the X-ray photography with the image intensifiers. Very recently, a strong demand is made for an enlarged type of stereoscopic radiography capable of performing the stereoscopic radiographing operation at a high speed with enlarged images.
In case of such an enlarged type stereoscopic radiography, the object under medical examination "P" is positioned, for instance, in between the two X-ray focal points R.sub.1, L.sub.1, and the X-ray detecting plane "I.sub.1 ", and also the distance D.sub.1 between these focal points R.sub.1 and L.sub.1 is reduced to approximately 35 mm (normally 63 mm).
However, if the distance "D.sub.1 " between two X-ray focal points "R.sub.1 " and "L.sub.1 " would be set to 35 mm, the triangle space "V.sub.1 " defined by the inner edges "XR.sub.1a " and "XR.sub.2a " of the pyramid X-rays "XR.sub.1 " and "XR.sub.2 " within which the first rectangular blades 16 and 17 are arranged would be narrowered, as compared with that of the normal distance "D.sub.1 " (=63 mm). As a result, there is a problem that the sufficient X-ray beam limiting operation could not be achieved by horizontally driving the conventional first rectangular (horizontally movable) blades 16 and 17.
Alternatively, if fixing type shield plates (not shown in detail) are arranged instead of the horizontally movable first rectangular blades 16 and 17, limitations are made in the above-described "SID" and common projection field.
The present invention has been made in an attempt to solve the above-described problems of the conventional X-ray beam limiting apparatus, and therefore, has an object to provide an X-ray beam limiting apparatus capable of perform an enlarged type X-ray stereoscopic photographing operation without restricting both "SID" and common projection field.